Pressure sensitive devices are used in many applications. Pressure-sensitive buttons, for example, find particular use in computer games in which the pressure-sensitive button is used to accelerate, brake, or steer an on-screen object such as a car. These pressure-sensitive buttons generally rely on mechanical parts that are subject to wear and, with repeated use, become uncalibrated.
Most recently, finger sensors have been used to emulate pressure-sensitive buttons. Some finger-sensor based systems recognize that the harder a finger is pressed on the sensor, the more ridges are captured by the sensor. However, these systems cannot recognize pressure changes when all of the ridges of a finger are initially captured by the sensor so that when additional pressure is applied the sensor cannot capture additional ridges. These systems are also ill-suited to determine changes in pressures when a finger is dry, making ridges harder to detect. In these systems, any changes in pressure are difficult or impossible to detect. All of these problems are exacerbated when using today's smaller “swipe” sensors, which only sense a small fraction of the fingertip area at any given time.
Other finger-sensor based systems recognize that when a finger is on the sensor, any pressure changes result in a corresponding change in the average pixel value of the image output by the sensor. Many of these systems, however, also use an automatic gain control (AGC) that artificially varies the average pixel value to keep it relatively constant and thereby maximize the sensor's dynamic range. Such systems cannot rely on the average pixel value to track changes in pressures.
One prior art system uses a fingerprint scanner to determine absolute pressure values. U.S. Pat. No. 6,400,836 to Senior, titled “Combined Fingerprint Acquisition and Control Device,” discloses estimating a “raw measure of force” applied by a finger on a scanner using an area of the finger on the scanner. Senior teaches determining the area either by counting the number of image pixels with a value above a background threshold or by finding the sum of the intensities of the “on” pixels above a threshold value. The system and method disclosed by Senior, for determining absolute pressures, is computationally expensive and because it attempts to determine absolute pressures, inherently inaccurate. Moreover, Senior teaches that a preferred embodiment of his scanner is “several times larger than that of currently available semiconductor fingerprint scanners.”
What is needed is a system for and method of determining pressure that do not rely on moving parts, ridge counts, or average pixel values but instead can be implemented easily and accurately in a solid state device, whether it be a full-sized or reduced-seized “swipe sensor.”